There are a wide variety of drum-based image forming devices that include one or more printheads. In one type of drum-based image forming device, the print media is carried by a rotating cylindrical drum past a printhead assembly that translates back and forth over the drum. Ink is deposited by the printheads directly onto the print media to create the desired image. The printheads include a plurality of very small nozzles and are typically associated with ink ejecting cartridges (or “pens”). Ink drops are fired through the nozzles by an ink ejection mechanism, such as a piezo-electric or thermal ejection mechanism, to create the desired dot pattern (or “image”).
The condition of the printheads is of paramount importance because of their direct effect on print quality. An improperly maintained printhead can become clogged and/or become the source of dot placement errors that reduce print quality. To that end, image forming devices that include printheads also typically include a printhead service station, which is located outside the print zone, to clean and protect the printheads. The printhead assembly moves from the rotating drum to the service station during non-printing periods and the shutdown process.
Spitting and wiping are two service station functions that may be performed during operation of the image forming device, albeit during non-printing periods, and also during start up and/or shutdown. Spitting clears clogs from the printhead by firing a number of drops of ink through each of the nozzles into a reservoir (or “spittoon”) that is part of the service station. Spittoons often include light sensors for drop counting. With respect to wiping, service stations are typically provided with an elastomeric wiper blade that wipes the printhead surface to remove ink residue, paper dust and any other debris that may have collected on the printhead. The wiping action, which is usually achieved through relative motion of the printhead and the elastomeric wiper blade, benefits from the moistening effect of spitting. Capping is another function that may be associated with service stations. The service station capping system seals the printhead nozzles to protect them from contaminants and prevent drying. This function is typically only associated with the shutdown process. The printhead nozzles are unsealed at startup.
Efforts are also continuously being made to address the dot placement error problems that can arise even when the printheads are properly maintained. For example, the alignment of the printhead assembly and rotating drum can be a source of dot placement errors. Such errors may, however, be substantially reduced by selecting and maintaining the optimum angular orientation of the printhead assembly relative to the rotating drum. Depositing ink directly from the printheads onto the print media can be another source of dot placement errors. One proposed solution to this problem is an image forming device in which ink is deposited by the translating printheads onto a rotating drum (or “print cylinder”), and then transferred from the print cylinder to the print media. An example of this type of imaging forming device is disclosed in commonly assigned U.S. application Ser. No. 09/571,647, which was filed on May 15, 2000, and is entitled “Digital Press and Method of Using the Same.”
Speed is another important printing consideration. Although service station functions such as spitting and wiping must be periodically performed, it is critical in many instances that downtime be minimized so that throughput can be maximized. The inventors herein have determined that moving the printhead assembly from the print zone to a service station and then back to the print zone is, however, a relatively slow process. It must be done carefully in order to insure that printhead errors are not introduced by variations in the orientation of the printhead assembly.
Accordingly, the inventors herein have determined that it would be desirable to increase the speed of service station functions such as, for example, spitting and wiping, without increasing the likelihood of dot placement errors in order to increase throughput while maintaining print quality.